Sparse-code multiple access (SCMA) is a non-orthogonal multiple access scheme that allows multiple devices, users, or user equipments (UEs) to share channel resources in the code domain. Potential transmit devices are allocated time and frequency resources, also referred to as resource units.
SCMA is a non-orthogonal multiplexing technique realized by the super-imposing of multi-dimensional codewords selected from codebooks which may be assigned to different layers, users or UEs. Instead of mapping coded bits to QAM symbols and repeating the same QAM symbols over multiple tones, as in code division multiple access (CDMA) and low density signature (LDS), coded bits are directly mapped to multi-dimensional codewords. One major benefit of SCMA codebooks is the shaping gain of multi-dimensional constellations. In SCMA, overloading is achievable with moderate complexity of detection thanks to the sparseness of SCMA codewords. SCMA can show noticeable gain over LDS especially for larger constellation sizes where the gain of constellation shaping is potentially larger. SCMA also provides system advantages due to its spreading and overloading capabilities. Interference whitening, open-loop user multiplexing and massive connectivity are some examples showing the benefit of SCMA from a system point of view.